Duplex powder metal bearing caps and method of making them

ABSTRACT

A main bearing cap (A′) made of powder metal has a body portion (Y) made from one powder metal material (Q), and a bearing arch portion (H), foot joint face portions (S) and/or wings (W) made of a different powder metal material (P). The material (Q) of the body portion (Y) is harder than the material (P) of the other portions (H, S, W), and the material (P) of the other portions (H, S, W) is relatively machinable. For the bearing arch portion (H), the machinability of the material (P) approximately matches the machinability of the bearing support structure (B) to which the bearing cap (A′) is assembled to produce a good quality bore and longer tool life during line boring. The bearing arch material (P) may be a bearing material.

This application is based on, and claims the benefit of, U.S. patentapplication Ser. No. 09/077,861 filed Jun. 4, 1998 and entitled “DUPLEXSPROCKET/GEAR CONSTRUCTION AND METHOD OF MAKING SAME”, now U.S. Pat. No6,148,685, which claimed the benefit of U.S. Provisional PatentApplication No. 60/008,696 filed Dec. 15, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to powder metal articles and their manufactureand in particular to a main bearing cap in which the body is made of ahigh strength powder metal material and other parts are made with adifferent more machinable powder metal material which is metallurgicallybonded to the body.

2. Discussion of the Prior Art

FIG. 1 illustrates a basic shape of a conventional main bearing cap(MBC). It is essentially a solid body with a semi-cylindrical recess andbolt holes for mounting. MBCs are used to retain the shell bearings andcrankshaft of internal combustion engines. This is accomplished bybolting the MBC A (FIG. 2) to the lower end of the engine block B (FIG.2). The semi-cylindrical recess C in the MBC corresponds to a similarsemicylindrical recess D in the engine block so as to form a round holewhen bolted together as shown in FIG. 3.

In multiple cylinder engines, there are multiple MBC/cylinder blockholes which are bored-out to a precise diameter E (FIG. 4) to acceptbearing shells F (FIG. 5), which in turn locate and retain thecrankshaft G (FIG. 5) in place. This boring operation is critical sincethe roundness and diametral precision have a significant effect onengine noise. A slightly oversize diameter allows the crankshaft tovibrate during operation. This is heard as a “rumble” that isunacceptable in modern passenger vehicles. A slightly undersize or outof round bore can cause binding of the crankshaft, preventing smooth lowfriction operation.

Traditionally, the cylinder block has been made from either grey castiron or an aluminum alloy, and the MBCs made from either grey cast ironor ductile cast iron (also called spheroidal or nodular cast iron). Inrecent years, a new material process combination has become commerciallyapplied, namely a sintered powder metal (P/,M) steel. A principal costadvantage of the P/M steel is the near net shape that can be achieved,which minimizes machining and associated costs of the product.

However, since the MBC has to be bored at the same time as the cylinderblock, there is a challenge related to the difference in machinabilityof the P/M steel and the block material (cast iron or aluminum alloy).This has lead to improvements in the P/M steel material machinability bywell known and published means that include additions of machinabilityaids to the P/M material. This has been beneficial, but not universallysuccessful in matching the MBC and block material machinability. Afurther issue is the ever increasing development of engine technologythat continues to try to obtain more power from smaller (lower weight)and faster turning engines to extend fuel economy and performancerespectively.

A natural extension of the MBC technology to handle this addedpower-density and higher loads is to raise the strength of the P/Msteel. This requires that the P/M steel be strengthened by some means,such as heat treatment, as is practiced in conventional steeltechnology. Heat treatment involves production of a stronger but alsomuch harder steel which is difficult to bore, and results in very shortlived and expensive cutting tooling. The short tool life meansinterrupted engine production on very costly automated machining lines.

Thus, a need exists to match the machinability of the recesses C and Dwhile maintaining a high strength, low weight bearing cap of near netpre-machined shape and dimension.

SUMMARY OF THE INVENTION

The invention provides a two material bearing cap made by powdermetallurgy. In one aspect, there is a thin layer of a more machinablematerial lining the half circular bore of the bearing cap, with astronger, harder material forming the majority and rest of the body ofthe bearing cap.

In this aspect, a bearing alloy composition may be used for the softermaterial in the bore arch region, which may be line bored to produce anintegral bearing surface, thereby eliminating the shell bearings whichotherwise are used. The integral bearing bore is only possible inbearing caps used in aluminum engine blocks. Whether the more machinablepowder material lining the half bore of the bearing cap is a bearingalloy or not, line boring is facilitated and tool life is prolonged bythe invention.

Also in this aspect, the material lining the half circular bore of thebearing cap is preferably chosen to match the machinability of the halfcircular bore of the engine block.

In another aspect, there may be provided a two material bearing cap madeby powder metallurgy where there is a thin layer of a softer material oneach joint face of the foot of the bearing cap, including where present,integral dowels, with a stronger harder material forming the majorityand rest of the body of the bearing cap. By forming the dowels of asofter material, they are more conformable to the counterbore in theengine block in which they are pressed, and better repressed into themduring the fit-up and installation of the crankshaft.

In another aspect, the bearing cap may be made with outboard wings forcross-bolting. These wings may be made from a softer, more machinablematerial, with a stronger harder material forming the majority and restof the body of the bearing cap.

Each of these aspects may be used alone, or in any combination with oneor more of the other aspects.

These and other features and advantages of the invention will beapparent from the detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art main bearing cap;

FIG. 2 is a plan view illustrating how a main bearing cap fits into anengine block;

FIG. 3 is a view like FIG. 2, with the cap bolted to the block;

FIG. 4 is a view like FIG. 3, but after the bearing bore has been boredout;

FIG. 5 is a view like FIG. 4, but with shell bearings F and crankshaft Ginstalled;

FIG. 6 is a view like FIG. 1, but of a main bearing cap incorporatingthe invention;

FIG. 7 is a view like FIG. 2, but illustrating a main bearing capincorporating the invention prior to line boring;

FIG. 8 is a view like FIG. 6, but after bolting and boring;

FIG. 9A is a sectional view of a die set-up, like FIG. 12, with a borelining powder being dispensed into the die cavity;

FIG. 9B is a detail view of a portion of FIG. 9A;

FIG. 10 illustrates the next stage of die filling in which the powder ofthe cap body is being dispensed into the die;

FIG. 11 illustrates the continuation of the filling step of FIG. 10,with the bore and leg punches lowered relative to the die housing;

FIG. 12 is a sectional view of a die model from the plane of the line12—12 of FIG. 13;

FIG. 13 is a top view of the die model of FIG. 12;

FIG. 14 is a sectional view of a die model from the plane of the line14—14 of FIG. 13;

FIGS. 15A-H are views similar to FIGS. 9-11 illustrating a die fillingsequence for forming softer wings in a bearing cap; and

FIG. 16 is a photomicrograph of the boundary of two materials made usingthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention offers a cost effective technical solution that couldonly be achieved by powder metallurgy. The invention provides alocalized area H (FIGS. 6 and 7) of machinable material in the P/M steelMBC bore which is largely machined away to leave a thin layer I (FIG.8). In one aspect of the invention, this material is introduced as aseparate powder P (FIG. 9), which is poured into the powder compactiondie cavity ahead of the regular P/M steel powder Q that forms the bulk(or body) of the MBC.

A technical challenge is to localize the machinable powder P in thedesired area. An insufficient thickness of powder P material at anypoint would lead to the boring tool hitting the harder P/M steelmaterial Q resulting in premature cutting tool failure. An excessthickness of powder material P results in lowering the overall strengthof the MBC since the residual P material area is not as strong as thehardened powder Q material. However, the latter condition is preferredsince the weakening effect will not be significant provided the relativeresidual thickness of the softer weaker material P is shallow comparedto the bulk material Q thickness.

It would be impossible to exactly match the thickness of P material tothe depth of the material bored-out since there are inherent dimensionaland locational variations in the boring process that result in differentthicknesses being removed from MBC to MBC. A common thickness ofmaterial bored-out from the MBC bore is imm (0.040 inches) with avariation from cap to cap of about 0.5 mm (0.020 inches). In such acase, therefore, up to 1.25 mm may be removed by boring. The thicknessof material P prior to boring, i.e., the thickness of layer H in FIG. 7,may then be targeted to be a minimum of 2 mm and a maximum of 4 mm. Thisleaves up to 2 mm thickness of material P after boring, i.e., thethickness of layer 1, which compares with typically a minimum of 50 mmof material Q thickness. Thus, the relative thickness of soft material Pto hard, strong material Q is only 4%. The strength reduction wouldtherefore be 4% of the difference in strength between the two materials.This is not a functionally significant reduction in strength.

To locate the powder P in the correct location, a press is used in whichthe individual compaction tooling members have independent motioncontrol, and is preferably a fully computer controlled compaction press.Also, a dual powder handling system is needed to keep the two powdersseparate until they are in the compaction die cavity, and also todeliver powder P in the correct location and to the correct depth,surrounded by powder Q.

Compaction tooling design and dual powder filling were initiallyresearched by use of a clear plastic die model which simulated theproduction compaction tooling, and where two different colored powderswere used to track the initial, transient and final location of the twopowders during the powder filling steps and subsequent pre-positioningof the tooling elements for compaction. FIGS. 12-14 show the clearplastic simulated compaction tooling 10. FIG. 13 shows in dashed linesthe powder P filling shoe 12 and the powder Q filling shoe 14 in dottedlines. The shoes 12, 14 in their simplest form are open bottom boxeswhich contain the powder P to be charged into the cavity in die 11. Theshoes 12, 14 are moved back and forth over the die cavity in the axialdirection indicated by arrow 16. This is the axial direction relative tothe bearing cap bore C. In the die cavity are positioned two leg punches20 and an arch punch 22 between them. The leg punches 20 are movable upand down together. The arch punch 22 is also movable up and down butindependently of the leg punches 20. Not illustrated in FIGS. 9A-14 arecore rods which would be used for forming the bolt bores through thelegs of the bearing cap, although in a production bearing cap thosebores would be formed by such core rods.

FIGS. 9A and 9B show the filling of the first powder P, which isrelatively machinable powder, into a die set like that shown in FIGS.12-14. FIGS. 10 and 11 show the distributions of powders P and Q duringpowder charging and after powder Q has been charged into the die cavityand the tooling elements moved to their final position, prior to thepowder compaction stage.

It can be seen that the powder P forms a fairly uniform layer around thebore area. This was the result of finding the optimum initial powderfilling pattern and mechanism which results in the desiredpre-compaction pattern shown in FIG. 11. This is achieved by positioningthe tool elements as shown in FIG. 9 and by using a powder P fillingshoe 12 of a specific width. This is shown*in FIG. 9, where powder Pforms a shaped profile in cross-section, viewed in the axial direction,which is a flat topped-triangular shape. This shape is dictated by the“angle of repose” of the powder P. This is the natural angle formed whenthe powder is poured in a narrow stream to form a cone shaped pile.

Using this approach, the width of the feed shoe 12 for powder P isadjusted to form the ideal initial pattern. Then a full width feed shoecontaining powder Q is passed over the die cavity such that powder Qfalls on top of powder P. filling in the spaces left by the slopes ofpowder P. Next, the tool elements (the three punches 20 and 22) aremoved to their pre-compaction position. It is during this motion thatthe shape of powder P changes to the ideal shape for subsequentcompaction.

Unfortunately, the two colored powders cannot be compacted in theplastic tooling since the plastic would crack under the pressure needed.Therefore, the lessons learned from this stage of development were takenand applied to actual production tooling which is made from highstrength tool steels (the punches 20, 22) and tungsten carbide (the die11). Using the pre-set tool element positions, samples were made fromtwo powders P and Q as described earlier.

After compaction and sintering, the final shape of the two compactedpowders P and Q was examined by sectioning the compact and observing theboundary as shown (100× magnification Nital Etch) in the photomicrographof FIG. 16. The result was that a layer of approximately 2 mm to 4 mmwas formed around the bore section, which was the intent of theinvention. Repeated trials resulted in sample compacts of MBCs whichwere sintered (the thermal process that metallurgically bonds the powderparticles together) and the resulting MBCs examined for microstructureintegrity at the bond zone between the powders P and Q. This was foundto be excellent.

In a variation of the invention, the first material P is also located onthe joint faces of the legs of the MBC. This is illustrated in FIGS.15A-H, in which the voids R are filled with the softer powder metal P.to form tapered bosses S (FIG. 15H, also referred to as integral hollowdowels). The tapered bosses S are molded onto the surface of each capfoot as disclosed in commonly owned International Patent Publication No.WO 97/42424, which is hereby incorporated by reference, to locate in acounterbore T (FIG. 2) on the mating bulkhead. Where the main body Y(FIG. 15H) of the MBC needs to be high strength/high hardness, arepressing step to calibrate the diametral size and angle of theintegral dowel may be impractical if the dowel is the same material asthe body. This is because strong hard P/M steel will not plasticallyyield to form the ideal geometry, but will either spring back to theoriginal shape, crack due to brittleness, or crack the repressing tool.Therefore, a layer of softer P/M material which includes the integraldowels, formed according to the present invention, enables therepressing process to be realized. Since this region of the MBC is incompression in service due to bolt loading, there is no detriment to thefunctional strength of the MBC.

In a third example of the invention, the MBC design includes extendedwings W (FIG. 2) that are bolted by bolts X (FIG. 3) horizontally to thecylinder block to provide rigidity and quietness. This design is called“cross bolted” and is gaining in popularity. Unfortunately, the designdemands that the material of the wings be machinable to accept a tappedhole which receives the bolt threads. Strong hard P/M steel with abainite or martensite structure is extremely difficult to drill and tap.Drill-bit life and tap life would be uneconomical. To overcome thisproblem, the invention proposes to make the wings W from the softermachinable powder material P. FIG. 15A shows the initial tool setposition ready to receive the soft powder P. The tool set includes borepunch 22, leg punches 20′, core rods 21 inside the leg punches 20′, andwing punches 23, all inside die 11′. FIG. 15B shows the powder P fillingdevice 12′, which is essentially an inverted open bottom box containingpowder P over the tool set. FIG. 15C shows the condition after thepowder fill box is withdrawn, thereby strickling (scraping excess powderoff) the surface. FIG. 15D shows the second powder fill box 14′containing powder Q over the tool set. FIG. 15E shows the tooling movedto the final filling position which draws powder Q into the tool set.FIG. 15F shows the condition where the second powder fill box 14′ haswithdrawn and strickled off the surface. The upper punch is also shownin FIG. 15G, ready to advance and compact the powders. FIG. 15G showsthe compaction completion, with upper punch 29 compacting the powders inthe die 11′, and FIG. 15H shows the dual material compact A′ afterejection from the tooling, with softer half bore H, softer bosses S andsofter wings W.

In a fourth application of the invention, the machinable material Pwould be made from a powder metal bearing material. In this case, itwould be possible to dispense with the traditional shell bearings, anduse the bearing cap bore layer material as the bearing surface for thecrankshaft. In this instance, the mating surface of the cylinder blockwould be the other half of the bearing. Since the cylinder block half ofthe bearing has much lower loads to bear (the combustion stroke isdirected away from this surface), then the parent block material wouldbe adequate for at least low to medium duty engines. There is asignificant cost savings in eliminating these half shell bearings.

The main bearing cap compact A′ is, of course, sintered to bind thepowder particles together. Any suitable sintering process may be used.The result is the finished or near finished main bearing cap A′, withthe powder metal distributions as illustrated in FIG. 15H. The bearingcap A′ is finished at this point unless some subsequent resizing, heattreating or surface finishing operations are needed to finish it.

In any of these constructions, the body Y material Q may be partially orfully hardened during or after sintering to a bainitic and/ormartensitic microstructure, but the soft material P is chosen such thatit does not respond to the hardening process remaining soft andmachinable.

For the particular materials to make a bearing cap of the invention, thesofter, more machinable powder material P may be a low to medium carbonpowder metal steel (e.g., 0-0.7% carbon) containing a machinability aidincluding but not restricted to one or more of copper above 3%,manganese sulfide up to 1%, boron nitride (non-cubic) up to 0.2%,magnesium silicate up to 1%, and calcium fluoride up to 1%. The harderbody material Q may be a 0.45-0.65% carbon, 0.45-0.65% phosphorus, 2-4%copper powder metal steel, or a 0.3-0.7% carbon, 0.3% copper steel.

Many modifications and variations to the preferred embodiments describedwill be apparent to those skilled in the art. Therefore, the inventionshould not be limited to the embodiments described, but should bedefined by the claims which follow.

We claim:
 1. In a bearing cap to be bolted to a bearing supportstructure so as to define a bearing bore between a bore arch of said capand said structure and in which bolt holes for securing said cap to saidstructure extend through feet of said cap and into said structure, saidcap having at least two of said feet, one said foot on each side of saidbore arch with at least one bolt hole extending through each said foot,the improvement wherein: said cap is made from at least two differentsintered powder metal materials and one of said powder metal materialsis harder than the other, and said harder material is selected from thegroup consisting of: a powder metal steel containing 0.45-0.65% carbon,0.45-0.65% phosphorus and 2-4% copper; and a powder metal steelcontaining 0.3-0.7% carbon and 0-3% copper.
 2. In a bearing cap to bebolted to a bearing support structure so as to define a bearing borebetween a bore arch of said cap and said structure and in which boltholes for securing said cap to said structure extend through feet ofsaid cap and into said structure, said cap having at least two of saidfeet, one said foot on each side of said bore arch with at least onebolt hole extending through each said foot, the improvement wherein:said cap is made from at least two different sintered powder metalmaterials and one of said powder metal materials is softer than theother.
 3. The improvement of claim 2, wherein the softer powder metalmaterial is positioned adjacent to said bore arch.
 4. The improvement ofclaim 3, wherein the machinability of said softer powder metal materialapproximately matches the machinability of said bearing supportstructure adjacent to said bore.
 5. The improvement of claim 3, whereinsaid softer powder metal material is also positioned on joint faces ofsaid feet.
 6. The improvement of claim 5, wherein integral dowels areformed on said joint faces of said feet, and said integral dowels aremade of said softer powder metal material.
 7. The improvement of claim2, wherein wings extend from ends of said bearing cap, and said wingsare formed from said softer powder metal material.
 8. The improvement ofclaim 7, wherein said softer powder metal material is also positioned onjoint faces of said feet.
 9. The improvement of claim 8, whereinintegral dowels are formed on said joint faces of said feet, and saidintegral dowels are made of said softer powder metal material.
 10. Theimprovement of claim 2, wherein said softer material is a low to mediumcarbon powder metal steel having 0-0.7% carbon and contains amachinability aid.
 11. The improvement of claim 10, wherein saidmachinability aid is one or more materials selected from the groupconsisting of: copper above 3%; manganese sulfide up to 1%; non-cubicboron nitride up to 0.2%; magnesium silicate up to 1%; and calciumfluoride up to 1%.
 12. In a bearing cap to be bolted to a bearingsupport structure so as to define a bearing bore between a bore arch ofsaid cap and said structure and in which bolt holes for securing saidcap to said structure extend through feet of said cap and into saidstructure, said cap having at least two of said feet, one said foot oneach side of said bore arch with at least one bolt hole extendingthrough each said foot, the improvement wherein: said cap is made fromat least two different sintered powder metal materials and one of saidmaterials is hardened and the other material remains soft.
 13. Theimprovement of claim 1 wherein said hardened material is hardened to amartensitic microstructure.
 14. The improvement of claim 1, wherein saidhardened material is hardened to a bainitic microstructure.
 15. In abearing cap to be bolted to a bearing support structure so as to definea bearing bore between a bore arch of said cap and said structure and inwhich bolt holes for securing said cap to said structure extend throughfeet of said cap and into said structure, said cap having at least twoof said feet, one said foot on each side of said bore arch with at leastone bolt hole extending through each said foot, the improvement wherein:said cap is made from at least two different sintered powder metalmaterials and said two materials differ in hardness, a body of saidbearing cap is made of the harder one of said materials and an exposedsurface of said bearing cap is made of the softer one of said materials.16. The improvement of claim 15, wherein said surface made of the softerpowder metal material is a bore arch of said bearing cap.
 17. Theimprovement of claim 16, wherein said softer powder metal material isalso positioned on joint faces of said feet.
 18. The improvement ofclaim 17, wherein integral dowels are formed on said joint faces of saidfeet, and said integral dowels are made of said softer powder metalmaterial.
 19. The improvement of claim 18, wherein wings extend fromends of said bearing cap, and said wings are formed from said softerpowder metal material.